A number of materials including metals such as aluminum, lead, magnesium, zinc, zirconium, titanium and silicon, for example, can be produced by electrolytic processes. Although individual processes may vary in some respects from one to another, each employs the use of an electrode which must operate in a highly corrosive environment.
An example of such a process for the production of metal is the well-known Hall-Heroult process (hereinafter referred to as the Hall process) for producing aluminum in which alumina dissolved in a molten fluoride salt bath is electrolyzed at temperatures from 900.degree. C. to 1000.degree. C. Typically the Hall process includes compartment which contains at least one anode disposed in the salt bath above a lower surface which acts as a cathode. There is an optimal gap between the bottom of the anode and cathode for producing aluminum. This gap is called the anode-cathode distance or "ACD." The optimal ACD depends on a variety of factors such as the size and shape of the bottom of the anode, voltage between the anode and cathode, and material used to make the anode. Typically, each of these factors remains constant. The ACD changes, however, because as the process reduces alumina to produce molten aluminum, a layer of molten aluminum collects between the lower surface and the salt bath. Because aluminum is conductive, the upper surface of the layer of molten aluminum acts as the cathode. Thus, as process creates aluminum, the elevation of the cathode increases and the ACD is reduced. The ACD also changes when molten aluminum is removed from the compartment for casting.
In the process as generally practiced today, carbon is used as the anode. In a typical operation of a Hall cell using carbon as the electrode, it is desirable that the carbon be in a block form. The carbon block is consumed during the electrolytic process and a large block or mass minimizes the frequency with which electrodes must be replaced. During the process, the carbon is oxidized to primarily form CO.sub.2 which is given off as a gas. The oxidation occurs mainly along the bottom surface of the anode, adjacent to the cathode. As the block is oxidized, the distance between the anode and the cathode increases. To adjust to anode cathode distance, the anode was typically mounted on a rod which could be moved vertically. This vertical adjustment accounted for both the rise in the elevation of the cathode and the reduction of the anode. Because the rise in elevation of the molten aluminum is typically not as great as the reduction in the size of the anode, the anode was usually being slowly lowered into the salt bath.
Despite the common usage of carbon as electrode material in practicing the Hall process, there are a number of disadvantages to its use. Carbon is consumed in relatively large quantities in the Hall process, approximately 420 to 550 kg per ton of aluminum produced. If prebaked electrodes are used, it may be seen that a relatively large facility is needed to produce sufficient electrodes to operate an aluminum smelter. Furthermore, to produce the purity of aluminum required to satisfy primary aluminum standards, the electrode must be relatively pure carbon, and availability and cost of raw materials to make the carbon are of increasing concern to aluminum producers.
Because of the disadvantages inherent in the use of carbon as an electrode, there has been a continuing search for inert or nonconsumable materials that can operate as an electrode with a reasonable degree of electrochemical efficiency and withstand the high temperature and extremely corrosive environment of the molten salt bath. One such material is a cermet material. Some cermet inert electrode materials are disclosed in U.S. Pat. Nos. 4,374,050, 4,374,761, 4,399,008, 4,455,211, 4,582,585, 4,584,172, 4,620,905, 5,794,112 and 5,865,980 and U.S. application Ser. No. 09/241,518, now U.S. Pat. No. 6,126,799, which are assigned to the assignee of this Application and which are incorporated by reference.
Cermet bodies are subject to cracking and damage. Therefore it is preferable to minimize moving the cermet anode in the salt bath. Because the cermet is not consumed, there is no longer a need to constantly lower the anode into the salt bath. The ACD still needs to be adjusted, however, due to the rising elevation of the molten aluminum which acts as the cathode.
Care must be taken, however, to avoid having a device in the molten aluminum and salt bath which creates a reservoir for impurities. When using a carbon anode, the compartment in which the molten aluminum forms is, generally speaking, a flat bottomed trough. This compartment may be maintained at an even temperature which is sufficiently hot enough to keep impurities from precipitating. If the compartment did not have this shape, pockets of cooler molten aluminum could form. Alumina and other impurities may precipitate in the cooler regions of asymmetric cell. This precipitate material, generally called "muck," is not desirable and must be removed from the molten aluminum.
There is, therefore, a need for a device to adjust the ACD which does not require moving the anode.
There is a further need for a device to adjust the ACD which provides a means for eliminating muck from the molten aluminum.